It goes without saying that Steampunk is already a sub-genre of something, that is, of speculative fiction. So, it’s more accurate to say that Steampunk’s sub-genres are not sub-genres of Steampunk at all, but rather derivatives of a genre called Cyberpunk.
Cyberpunk is the literary predecessor of Steampunk. The term was coined in the 1980’s as a label for stories dealing with punk teenagers during the Information Age. Most of the time these stories took on a dic setting in the future, where computer technology has invaded every facet of humanity’s daily life. The stories often have great emphasis on virtual worlds and cyberspace. Famous examples of Cyberpunk include films like Blade Runner and The Matrix.
The derivatives of Cybperpunk emerged when writers began to explore other avenues of futurism and retro-futurism; creating worlds dependent on one technology. In Steampunk’s case, it was steam. The other derivatives include:
Dieselpunk – a style which showcases the aesthetics that were popular during World War I and World War II. This can include pulp, film noir, art deco, and wartime pinups. If Steampunk relies mostly on steam technology, then it’s safe to say that Dieselpunk might give emphasis to combustion. Famous examples of this genre include The Rocketeer and Avatar: Legend of Korra.
Atompunk – projects the pre-digital age of 1945-1965, or more specifically, the sensibilities of the Cold War era. This includes Communism, the Space race, atomic technology, and nuclear apocalyptic scenarios. One famous example of this style includes the Fallout video game series.
Biopunk – a futuristic derivative that emerged during the 1990’s, dealing mostly with the unintended consequences of biotechnology and genetic manipulation. Many Biopunk stories are like modern retellings of Frankenstein, in that they deal with the theme of man’s unwieldy power over the forces of nature. A great example of Biopunk is the Resident Evil video game series, which seems to have all the staples of the genre: human experimentation, evil megacorporations or governments out to turn a profit, and horrible monsters and/or viruses which threaten to wipe out humanity. A lot of modern zombie apocalypse stories can be categorized as Biopunk.
Stonepunk – dealing with the Stone Age time period as well as it’s technology, though usually with anachronistic elements. The Flintstones is a famous example of this sub-genre.
Clockpunk – a derivative similar to Steampunk, except it deals with Renaissance-era technology.
There are plenty of other Cyberpunk derivatives out there, but most of them are fairly young in comparison to these. Contemporary genres like Nowpunk and Elfpunk might see greater exposure in the future.